Ion thrusters and plasma accelerators produce high velocity streams of ions and electrons that impart momentum to spacecrafts. The propulsion of satellites using plasma streams is in increasing demand in order to improve the performances of satellites. An important limiting factor is the depletion of propellant, essential for in-orbit maneuvers, which eventually might force early satellite retirement. These orbit corrections and changes of orientation compensate the small variations produced in the periodic motion of satellites by the gravitational forces of the sun, the moon, as well as by the irregular distribution of the Earth's mass. Replacing the usual chemical rocket engines with plasma-based propulsive systems, characterized by high propellant exhaust speeds and large values of specific impulse, increases the operational lifetime of satellites.
In plasma propulsion devices the neutral gas employed as propellant is introduced into a lengthwise cavity called discharge or plasma chamber. The latter is made of different shapes and materials and has an open side to allow ion outflow. The plasma composed of electrons and ions is produced inside the discharge chamber by neutral gas atom collisional ionization by electrons emitted from active cathodes. As used herein, the term “active cathode” refers to electron emitting electrodes having substantial emission current densities, roughly over 10−2 A/cm2. These devices, such as hollow cathodes or thermionic electron emitters, could be operated as electron sources. Otherwise, “passive cathodes” also are negatively polarized electrodes but having much lower or negligible electron emission current densities, typically below 10−3 A/cm2.
The ions from this plasma flow through the open side of the discharge chamber and are accelerated by different physical mechanisms. The thrust is imparted to the spacecraft by the plasma stream created when electrons are added to the high energy ion beam to neutralize space charge effects. This plasma stream composed of electrons and ions could also be used in material processing applications by directing the energized ion outflow over the surface of materials in order to modify their physical properties.
Electrostatic plasma accelerators can be roughly classified as gridded ion engines, Hall effect thrusters and multi-stage plasma accelerators. Only gridded ion engines deliver variable or modulated thrust by electrical control of the speed of the plasma stream exhaust, holding other operational parameters of the thruster constant. Such control or modulation of throttle is essential for orbital maneuvers and/or flight formation of satellites. Other propulsive systems deliver a fixed thrust essentially determined by plasma discharge parameters such as the current of ionizing electron flow, neutral gas mass flow rate, etc.
Gridded ion engines produce the electrostatic acceleration of ions extracted from a plasma and are disclosed in U.S. Pat. No. 4,466,242, U.S. Pat. No. 3,956,666, U.S. Pat. No. 3,969,646 or EP 073380061. The conductive walls of the discharge chamber constitute the anode of an electric discharge, triggered by ionizing electrons emitted by a first active cathode placed inside.
A minimum set of two multi-aperture parallel grids are disposed in front of the open side of the discharge chamber for the extraction and acceleration of the ion beam. The first grid, currently called extraction or screen grid, is electrically connected to the active cathode and allows the ions to pass through its open spaces. The second grid is biased to high voltages and accelerates the flow of outgoing ions. Finally, a second active cathode disposed beyond the grids provides electrons to neutralize space charge effects of the ion beam and produces the plasma stream exhaust. The acceleration voltage of the second grid allows modulating the speed of outflowing ions and therefore the delivered thrust by the ion engine.
A third decelerator grid described in U.S. Pat. No. 5,369,953 and U.S. Pat. No. 5,559,391 is used to avoid charge exchange collisions between ions and neutral atoms and also reduces erosion of the accelerator grid by the electron backflow from the second active cathode employed for ion beam neutralization. This protective grid extends the lifetime of the system. Additional improvements of this basic scheme have been disclosed, such as higher ion production rates by means of multi-cusp magnetic fields inside the discharge chamber as in U.S. Pat. No. 4,466,242, or improved accelerator grids in US 2010/0212284A1.
Gridded ion engines require a minimum of two active cathodes and therefore a number of power supplies that increase the complexity of the electrical system, as well as the electric power consumption. Thermal control also becomes an issue because of the high operational temperatures of active cathodes, typically over 2000 K. Different thruster elements are heated up to high temperatures by the released infrared radiation. Additionally, the high bias voltages required for ion beam acceleration, typically of a few kilovolts, also give rise to sparking and electric arcing between the grids, also subjected to both thermal stresses and charged particle bombardment. All these factors reduce the lifetime of gridded ion engines.
The principle of the plasma accelerator called Hall thruster is the electrostatic acceleration of ions without the grids required by ion thrusters. This configuration, described in EP 0541309A1 or U.S. Pat. No. 8,468,794, is simpler and needs fewer power supplies to operate. The discharge chamber is a cylindrical ring-shaped cavity made of a non-conductive or ceramic material extended along its axial direction. The passive annular anode is located at its closed bottom, in the opposite direction to the open side of this chamber intended for ion beam exit. An intense radial magnetic field inside the plasma chamber is produced by a first pole configuration about the central axis, which is surrounded by the plasma chamber. The second pole configuration surrounds the plasma chamber on the outside, as a magnetic counter-pole.
The only active cathode of Hall thrusters is located outside the discharge chamber, close to the ion beam exit. The flux of emitted electrons spreads in two directions, inwards to the discharge chamber and also along the direction of the outgoing ion beam for space charge neutralization.
The radial magnetic field into the annular discharge chamber inhibits the axial electron motion confining the electrons along ring-shaped drift paths. These radially confined electrons ionize the neutral gas introduced into the annular discharge chamber. Additionally, the high voltage applied between the active cathode and the anode produces an electric field inside the discharge chamber along its axis of symmetry that drives the ions towards its open end. This combination of radial magnetic field and axial electric field produce a fast circulating electron current around the axis of symmetry of the thruster with a slow drift towards the anode. The ions are less affected by the magnetic field and are accelerated by the axial electric field originated by the negative charge density, provided by the concentration of electrons at the open end of the thruster.
The axial thrust delivered by Hall thrusters is un-modulated and essentially determined by the physical plasma discharge parameters. Modifications of this basic scheme with more involved magnetic field configurations and improved schemes have been disclosed in U.S. Pat. No. 5,847,493, U.S. Pat. No. 7,543,441B2, U.S. Pat. No. 5,845,880 or in US 2012/0206045A1
The ionization efficiency and specific impulse of Hall thrusters are lower than those achievable by ion engines. The strong magnetic field also introduces rotational oscillations, waves and instabilities in the plasma inside the annular discharge chamber with frequencies roughly in the range from 1 kHz up to tens of MHz. In addition, ion and electron bombardment deposits over the surface of the dielectric walls of the discharge chamber important amounts of energy, in particular at the exit section. The subsequent erosion and degradation of these dielectric walls ultimately determines the lifetime of Hall thrusters.
The so-called multi-stage plasma accelerator configuration described in DE 19828704A1, U.S. Pat. No. 6,523,338B1, US 2003/0048053A1 or U.S. Pat. No. 7,084,572B2 also delivers a constant un-modulated axial thrust. As for Hall accelerators, the cylindrical discharge chamber is made of non-conductive materials and extends lengthwise with an open end for ion beam exit. The electron source is disposed in front of such open side, and also a fraction of the emitted electrons is guided into the discharge chamber. The active cathode therefore provides electrons for outgoing ion beam neutralization and also for neutral gas collisional ionization.
This discharge chamber is surrounded by ring-shaped permanent magnets with alternate polarities disposed along its longitudinal axis. They confine the electrons along a spatially periodic magnetic field along the longitudinal direction, whereas the motions of the more massive ions of the working gas are less affected. The anode is placed at the closed end of this tubular plasma chamber and additional ring-shaped intermediate electrodes are disposed inside along its longitudinal direction.
These intermediate electrodes are intended for ion acceleration and are electrically connected to increasing electric potentials. Consequently, the electrons are essentially confined close to the axis of the discharge, whereas the ions are accelerated in the direction towards the open end of the discharge chamber. Additionally, the electric field also accelerates the electrons from the active cathode downstream towards the anode. This combination of electron confinement by the magnetic field and acceleration by the local electric field increases the ionization rate of the working gas inside the discharge chamber. As with Hall plasma accelerators, ion outflow is basically determined by physical plasma discharge parameters, which control the delivered axial thrust.
The plasma streams produced by multi-stage thrusters are less collimated than those of Hall thrusters and gridded ion engines. Ion confinement is better than in Hall thrusters, except at magnetic field cusp positions along the longitudinal axis of the discharge chamber, which reduces the wear of its dielectric walls. Additionally, the radial symmetry of the magnetic field in multi-stage thrusters produces a spoke rotation regime in the plasma column along the perpendicular direction to the electric and magnetic fields, with typical frequencies of 15-35 kHz, which might cause turbulent regimes.
The plasma thruster with a multi-cusp magnetic configuration disclosed in US 2012/0167548 A1 or in EP 2414674A1 is essentially intended to provide non-axial thrust by changing the plasma jet exhaust. The cylindrical discharge chamber also having an open and a closed side is surrounded by a plurality of magnets located in the plane perpendicular to its axis of symmetry. The anode is located at the closed end of the discharge chamber and the active electron source is placed in front of its open side for working gas ionization and neutralization. In this magnetic configuration, the pole of each magnet is disposed adjacent to the like pole of the adjacent magnet and at least one of them is an electromagnet, arranged to produce a variable magnetic field.
This configuration produces constant un-modulated thrust along the axial direction depending on physical plasma discharge parameters. The control of the variable magnetic field at the open end of the discharge chamber partially deflects the ion outflow from the axis of symmetry, adding a non-axial thrust component. Alternatively, this purpose could be also achieved by means of additional passive electrodes combined with permanent magnets or electromagnets disposed outside the discharge chamber.
Variations in time and transients of the variable magnetic field introduce fluctuations of charged particle currents in the plasma. This introduces oscillations in the deflection of the plasma beam exhaust in the direction perpendicular to the axis of symmetry of the thruster that are difficult to control and therefore the delivered thrust.